The present invention relates to a control apparatus for an AC generator, and more particularly, to a control apparatus for a vehicular AC generator in which the required capacitance of a smoothing capacitor, which serves to smooth a generation signal provided by the generator for controlling an indicator connected through a power generation detector and a rectifier to the armature coil of the generator, can be reduced, and in which a storage battery for supplying a current to the field coil of the generator through the rectifier is made difficult to discharge through the rectifier and the power generation detector when diodes as used in a rectifier are deteriorated for some reason.
A typical example of an AC generator having a known control apparatus is illustrated in FIG. 3. In this figure, an AC generator in the form of a vehicular AC generator, generally designated by reference numeral 1, includes an armature coil 101 in the form of a three-phase armature coil comprising three coil elements arranged in a star-like manner and connected at their one end to each other, and a field coil 102.
A rectifier in the form of a full-wave rectifier, generally designated by reference numeral 2, is connected to the armature coil 1 for rectifying the outputs thereof. The full-wave recifier 2 has an output terminal 201, a ground terminal 202 connected to ground, and three input terminals 203 each connected to the other end of a corresponding coil element of the armature coil 1. The full-wave rectifier 2 also includes three pairs of rectifying diodes, each pair comprising two diodes serially connected to each other at a junction or input terminal 203. The three pairs of serially connected diodes are connected in a parallel relation with each other at their opposite ends to the common output terminal 201 and the ground terminal 202, respectively.
A voltage regulator, generally designated by reference numeral 3, is connected to the field coil 102 of the AC generator 1 and to the output terminal 201 of the rectifier 2. The voltage regulator 3 includes a pair of voltage dividing resistors 301, 302 serially connected to each other between the output terminal 201 of the rectifier 2 and ground, a Zener diode 303 having a cathode connected to a junction between the resistors 301, 302, a control transistor 304 having a base connected to an anode of the Zener diode 303 and an emitter connected to ground, a base-current supplying resistor 305 having one end thereof connected to a collector of the transistor 304, a switch 306 in the form of a power transistor having a base connected to a junction between the collector of the control transistor 304 and the base-current supplying resistor 305, an emitter connected to ground and a collector connected to the output terminal 201 of the rectifier 2 through the field coil 102 of the AC generator 1, and a suppression diode 307 having a cathode connected to the collector of the power transistor 306 and an anode connected to the output terminal 201 of the rectifier 2.
A storage battery 4 is connected between the output terminal 201 of the rectifier 2 and ground. The storage battery 4 is also connected at a positive terminal or electrode thereof through a key switch 5 to the other end of the base-current supplying resistor 305 of the voltage regulator 3 and directly to one end of the voltage dividing resistor 301.
A power generation detector 6 is connected between the key switch 5 and the AC generator 1, and between the key switch 5 and the voltage regulator 3 for detecting the operating state or power generation of the AC generator 1. The detector 6 comprises a smoothing resistor 601 having one end thereof connected to one of the input terminals of the rectifier 2 or one end of one of the coil elements of the armature coil 101, a reverse-current checking diode 602 having an anode connected to the other end of the smoothing resistor 601, a smoothing capacitor 603 connected between a cathode of the reverse-current checking diode 602 and ground, a smoothing resistor 604 having one end thereof connected to the cathode of the reverse-current checking diode 602, a control transistor 605 having a base connected to the other end of the smoothing resistor 604 and an emitter connected to ground, a base-current supplying resistor 606 connected between a collector of the control transistor 605 and the key switch 5, and a detector switch 607 in the form of a power transistor having a base connected to a collector of the control transistor 605 and an emitter connected to ground.
An indicator 7 in the form of an indicator lamp is connected between the power transistor 607 and the key switch 5.
In the above description, the rectifier 2, the voltage regulator 3, the storage battery 4, the key switch 5, the power generation controller 6 and the indicator lamp 7 together constitute a known control apparatus.
In operation, when the key switch 5 is first turned on, the AC generator 1 is still held inoperative or does not generate electric power. Thus, at this time, the generator 1 does not supply a generation signal in the form of a current to the power generation detector 6, so the control transistor 605 is held non-conductive. On the other hand, with the turning on of the key switch 5, a current is supplied from the storage battery 4 through the base-current supplying resistor 606 to the power transistor 607 which is thereby made conductive, turning the indicator lamp 7 on. As a result, it is indicated that the generator 1 generates no electric power.
Also, at the time of the key switch 5 being turned on, the storage battery 4 does not have a high voltage sufficient to make the Zener diode 303 conductive, so the control transistor 304 is non-conductive or turned off. Thus, at this time, a current begins to flow from the storage battery 4 to the base of the power transistor 306 through the key switch 5 and the base-current supplying resistor 305, making the power transistor 306 conductive. As a result, a current is supplied from the storage battery 4 through the now conductive power transistor 306 to the field coil 102 of the AC generator 1 to energize it. Subsequently, when an unillustrated engine is started under this condition, the AC generator 1 is driven to rotate by the engine so that it begins to generate electric power and a generation signal in the form of a current. The current thus generated by the generator 1 is supplied from the armature coil 101 to the power generation detector 6, whereby the control transistor 605 of the detector 6 is made conductive, turning the power transistor 607 off. As a result, the indicator lamp 7 is also turned off, thus indicating the start of power generation of the generator 1.
However, as long as the output power of the AC generator 1 is less than a predetermined value, both the Zener diode 303 and the control transistor 304 remain non-conductive, so the power transistor 306 continues to be held conductive. Accordingly, the field current supplied from the battery 4 to the field coil 102 increases, raising the output voltage of the AC generator 1. When the generator output voltage exceeds the predetermined value, the Zener diode 303 becomes conductive so that a current is supplied from the battery 4 to the base of the control transistor 304 through the now conductive Zener diode 303. As a result, the control transistor 304 is turned on so that the current supply from the battery 4 to the base of the power transistor 306 is stopped, thus turning it off. Consequently, the current supply to the field coil 102 decreases to reduce the output power of the generator 1. With a repetition of the above operations, the output voltage of the generator 1 is regulated to the predetermined value under the action of the voltage regulator 3.
With the known control apparatus as constructed above, the generation signal output from the armature coil 101 contains an AC component, which is required to be smoothed. In this case, since the generation signal from the generator 1, after being smoothed, is input to the emitter-grounded control transistor 605, it is necessary to increase the capacitance of the smoothing capacitor 603 in order to increase the smoothing effect or a CR time constant. This unnecessarily results in an increase in the manufacturing cost.
In addition, the relatively low input impedance of the power generation detector 6 for the generation signal requires an accordingly low resistance of the smoothing resistor 601. Accordingly, in the event that there occurs a current leakage due to the deterioration of any of the diodes of the rectifier 2 at the side of the output terminal 201 thereof, a relatively large current flows from the storage battery 4 to ground by way of the deteriorated diode of the rectifier 2, the smoothing resistor 601, the reverse-current checking diode 602, the smoothing resistor 604 and the base-emitter of the control transistor 605 even during the inoperative condition of the AC generator 1, i.e., when the key switch 5 is turned off. Thus, the storage battery 4 is easy to discharge through the detector 6 upon a failure in the rectifier 2.
Moreover, when the storage battery 4 is fully charged by a continued operation of the generator 1 and held, for an extended period of time, at a voltage level which is higher than the predetermined value to which the voltage regulator regulates the output voltage of the generator 1 (this is called "a floating phenomenon" of the storage battery 4), the control transistor 304 is continuously conductive to hold the power transistor 306 non-conductive with the result that the voltage level of the generation signal from the generator 1 gradually falls or decreases. When the generation signal falls below a prescribed level, the control transistor 605 becomes non-conductive whereby the power transistor 607 is turned on. As a result, the indicator lamp 7 is lighted, incorrectly indicating that the generator 1 is out of operation.